Vacuum systems are used in scientific research, the semiconductor industry, many analytical instruments, and various similar applications. Processes or experiments that require high or ultra-high vacuum (UHV) typically employ all-metal vacuum joints or seals. Referring to FIG. 1, such a joint typically comprises a flange 10 that includes an annular recess 15 and an annular knife edge 20. The flange 10 is intended for mating with another like flange 10 separated by a soft, metallic gasket 25. The opposing knife edges 20 are pressed into the gasket 25 by tightening bolts (not shown) forming the UHV seal, e.g. in accordance with the teachings of U.S. Pat. No. 3,208,758. Non-metal gaskets may be used in certain applications. Most embodiments utilize circular flanges; however, the flanges do not need to be circular.
Prior art flanges 10, seen in FIGS. 2A-2C and 3A and 3B, utilize a stepped internal diameter 30, and weld easily only to tube 35 or to tube-like projections machined onto another shape 40. Frequently, an extension tube 35 is welded into a cylindrical main vacuum chamber 70 and a flange 10 is welded onto the extension tube 35. The weld between the extension tube 35 and the cylindrical main chamber 70 is a difficult, non-planar weld which can cause inaccuracy in the flange's positioning. For spherical main chambers 65 and centered flange ports 10 the weld is a simple planar weld, but an extension tube 35 is still required. The extension tube 35 moves the port opening away from the surface of the sphere which interferes with the access and visibility of the interior of the main chamber. Also, the weld of the extension tube 35 to the interior of the flange 10 decreases the possible working diameter of the flange's bore 45, and can only be accessed from the interior of the chamber for smaller sized flanges. This requires that at least one port on the chamber be large enough to accommodate a welding torch, restricting the minimum size of the chamber.